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Deep Time and Ancient Life in the Columbia Basin Rolf Ludvigsen, Palaeontologist

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Deep Time and Ancient Life in the Columbia Basin Rolf Ludvigsen, Palaeontologist Living Landscapes Deep Time and Ancient Life In The Columbia Basin by Rolf Ludvigsen, Palaeontologist Copyright 1999 Royal British Columbia Museum Deep Time And Ancient Life In The Columbia Basin Rolf Ludvigsen, Palaeontologist Introduction The term "deep space" alludes to the unimaginably vast distance between stars meas- ured in millions of light years. The American writer John McPhee coined a parallel term "deep time" to capture the full sweep of geologic time that is scaled off in billions or millions of years. Both defy understanding by the human mind. In the corner of the Cordillera called the Columbia Basin, deep time extends back a thousand million years. Geologists assign these rocks to an older Cryptozoic (or Precambrian) Eon, and a younger Phanerozoic Eon. The boundary is approximately at 650 million years ago. The Phanerozoic Eon has a rich fossil record and the succes- sion of fossils is used to divide these strata into eras, systems and series. The Colum- bia Basin includes fossil-bearing rocks of Palaeozoic, Mesozoic and Cenozoic age, but the best preserved and most informative fossils come from Cambrian strata (550 to 500 million years old) and Ordovician strata (500 to 440 million years old) that are exposed in the eastern part of the Columbia Basin. This paper explores a few of these fossil sites, their discovery and their significance. Royal British Columbia Museum 675 Belleville Street 1-888-447-7977 1 Victoria, British Columbia (250) 387-3701 CANADA http://www.royalbcmuseum.bc.ca Living Landscapes Deep Time and Ancient Life In The Columbia Basin by Rolf Ludvigsen, Palaeontologist Copyright 1999 Royal British Columbia Museum Geologic time scale of the Phanerozoic Eon, divided into era, systems and series, and measured in millions of years. Royal British Columbia Museum 675 Belleville Street 1-888-447-7977 2 Victoria, British Columbia (250) 387-3701 CANADA http://www.royalbcmuseum.bc.ca Living Landscapes Deep Time and Ancient Life In The Columbia Basin by Rolf Ludvigsen, Palaeontologist Copyright 1999 Royal British Columbia Museum Geology The Columbia Basin is that large, triangular mountainous region in southeastern British Columbia that is drained by the Columbia and Kootenay rivers. It includes the Main Ranges of the Rocky Mountains to the east, the Purcell Mountains, the Selkirk Moun- tains and, to the west, the Monashee Mountains. The Columbia Basin does not corre- spond to a coherent geologic region -- in fact, it is crossed by a fundamental geologic boundary that separates Laurentia, ancient North America, from exotic terranes that collided with Laurentia during the Mesozoic. The strata making up the Rocky Mountains are largely limestones and shales of Palaeozoic age, deposited as a thick apron on the seaward side of Laurentia. Even though these rocks are faulted and upthrust, they have not been significantly altered or metamorphosed and their contained fossils are generally well-preserved. The rocks of the Purcell and Selkirk mountains are mainly sandstones and grits of late Cryptozoic (late Precambrian) age. These rocks are commonly highly deformed and metamor- phosed, and locally they are intruded by younger granites. They were deposited on the margin of ancient North America. The Monashees and related mountain ranges in the western Columbia Basin are made up of a crazy quilt of rocks -- including exotic terranes of Late Palaeozoic and Mesozoic volcanics and sedimentary rocks, as well as large masses of granites. During the Jurassic, Laurentia, with its thick packages of late Cryptozoic and Palaeozoic strata, collided with large exotic terranes that had formed out in the palaeo- Pacific Ocean during the late Palaeozoic. This collision pushed the entire package of Palaeozoic rock eastward in a series of overlapping faults to form the Rocky Moun- tains. The late Cryptozoic rocks were squeezed and faulted up to form the structures now seen in the Purcell and Selkirk mountains. The colliding terranes on the west were shoved onto the granitic basement, making the crust much thicker, altering and heating it, and causing the melted rocks to be intruded as large granitic masses. The geology of the entire Cordillera is covered in considerable detail by Gabrielse and Yorath (1992). Yorath (1997) is a more accessible account of the geology of the Rocky Mountains that is written for a non-technical audience. (Generalized geologic map of the Columbia Basin.) Royal British Columbia Museum 675 Belleville Street 1-888-447-7977 3 Victoria, British Columbia (250) 387-3701 CANADA http://www.royalbcmuseum.bc.ca Living Landscapes Deep Time and Ancient Life In The Columbia Basin by Rolf Ludvigsen, Palaeontologist Copyright 1999 Royal British Columbia Museum Generalized geologic map of the Columbia Basin. Royal British Columbia Museum 675 Belleville Street 1-888-447-7977 4 Victoria, British Columbia (250) 387-3701 CANADA http://www.royalbcmuseum.bc.ca Living Landscapes Deep Time and Ancient Life In The Columbia Basin by Rolf Ludvigsen, Palaeontologist Copyright 1999 Royal British Columbia Museum Episodes of Cambrian and Ordovician Life The Trilobite Beds After the last spike was driven at Craigellachie in 1885, the General Manager of the Canadian Pacific Railway, Cornelius Van Horne, ordered the construction of a series of dining chalets along the main line through the mountains. These chalets were to service the flood of tourists who were expected to come west on the CPR to experi- ence the splendour of mountain wilderness (and diminish the massive debt load from the construction of the mountain sections of the CPR). Some of the workmen building the chalet in Field, Mount Stephen House, spent their free Sundays scrambling across the steep slopes of the surrounding mountains. One day a group of carpenters with now-forgotten names came across abundant "stone bugs" in shales high on Mount Stephen above Field. The authorities were promptly notified and, later in the summer of 1886, a couple of government men (a geologist and a surveyor) followed the car- penters up the mountain to make large collections of these well-preserved fossils. The collections were sent to different palaeontologists who recognized that these were some of the best Middle Cambrian trilobites known anywhere. One of these palaeon- tologists was Charles D. Walcott who, 20 years hence, was to come to Field to collect more fossils from the Trilobite Beds, as the locality on Mt Stephen came to be known. Walcott returned to the Canadian Rocky Mountains and, in 1909, he discovered the soft-bodied Burgess Shale fossils at a nearby locality. Trilobites, especially large complete trilobites, are rather uncommon fossils at virtually all Cambrian sites. A first-time visitor to the Trilobite Beds is startled by the realization that virtually every slab on this mountainside contains large, complete and articulated trilobites. Many are gleaming black and stand out conspicuously from the buff-col- oured shale, others are evident by being slightly raised from the matrix in bas-relief. Ogygopsis, the most common trilobite here, has the outline and size of a kid's shoe- print. A close second is Olenoides, which is easy to recognize because it has a spiny tail. A few other trilobites are found, and non-trilobite fossils are not uncommon. Par- ticularly interesting is the curved and segmented fossil called Anomalocaris that was considered to be the body of a crustacean. The sheer abundance of large fossils in the Trilobite Beds attracted many visitors to this site in Yoho National Park. To conserve the site, access was initially restricted to small, guided parties. But regrettably, because visitors walking across a mountainside thickly covered by fossiliferous slabs have, over the years, inadvertently damaged this important site, Parks Canada has recently found it necessary to close off all access to the Trilobite Beds. Royal British Columbia Museum 675 Belleville Street 1-888-447-7977 5 Victoria, British Columbia (250) 387-3701 CANADA http://www.royalbcmuseum.bc.ca Living Landscapes Deep Time and Ancient Life In The Columbia Basin by Rolf Ludvigsen, Palaeontologist Copyright 1999 Royal British Columbia Museum Complete trilobites (Ogygopsis and Olenoides) from the Trilobite Beds. Photo by L. MacKenzie McAnally, Univ. of Victoria. Anomalocaris was thought to be the body of a crustacean. Photo by B. Chatterton, Univ. of Alberta. Royal British Columbia Museum 675 Belleville Street 1-888-447-7977 6 Victoria, British Columbia (250) 387-3701 CANADA http://www.royalbcmuseum.bc.ca Living Landscapes Deep Time and Ancient Life In The Columbia Basin by Rolf Ludvigsen, Palaeontologist Copyright 1999 Royal British Columbia Museum The Burgess Shale On the last day of August, 1909, Charles D. Walcott and his field crew were wrapping up work for the season. They were travelling by horseback along a well-established trail on the ridge between Mount Wapta and Mount Field above Emerald Lake, on their way to the comfort of Mount Stephen House in the village of Field. The lead horse was halted by a slab of shale that had fallen across the narrow trail. Before the crew could dislodge it, Walcott caught sight of some faint and shiny, but well-defined, out- lines of fossils. He knew Cambrian fossils better than anyone in the world, and imme- diately recognized that these were unknown types of arthropods. The non-mineralized cutile of these animals was clearly evident, but they also displayed, in astonishing de- tail, their soft-bodied anatomy — spindly legs, antennae, comblike gills, even guts and muscles. Walcott also spotted segmented worms and sponges among the fossils as well as a few familiar trilobites, establishing that these strange fossils were of Middle Cambrian age. Walcott knew that he had made the discovery of a lifetime and, from then on, palaeon- tologists and biologists would never again consider Cambrian life to be primitive, simple or impoverished. Walcott returned to the site now known as the Burgess Shale every summer until 1914.
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